Several industrial processes are based on the electrolysis of aqueous salt solutions. One important application is that of the electrolysis of sodium chloride brine to produce sodium hydroxide and chlorine. In this electrolysis, as in others involving electrolysis of aqueous salt solutions, one method is to separate the anolyte and the catholyte compartments of the cell using a porous separator. Hereinafter the term "diaphragm" will be used for a porous separator which permits electrolyte to pass through without any significant change in composition.
In this method hydrogen and caustic soda are produced at the cathode which is within the catholyte compartment of the cell while chlorine is produced at the anode which is within the anoltye compartment. The salt brine passes through the diaphragm from the anolyte to the catholyte compartment. The cathodes are usually of iron screening while the anodes are of graphite or platinized titanium. The diaphragm is usually aspestos.
Diaphragm cells require a flow of solution sufficient to insure that the back-diffusion of sodium hydroxide into the anolyte is avoided or minimized. This is necessary to avoid chlorate in the anolyte and loss of current efficiency. When using the minimum amount of brine flow within the diaphragm to avoid chlorate formation, only about half of the sodium chloride is converted. The spent anolyte must then be evaporated to concentrate the caustic soda and to crystallize the salt. Finally the salt is filtered or centrifuged from the caustic solution. It is the usual practice to purify the brine feed to the process to reduce the content of impurities which clog the diaphragms; also it is usual to renew the diaphragms at regular intervals.
Attempts have been made to build diaphragm cells using permselective membranes in place of the diaphragms. This does not provide a solution to the principal problem. All of the membranes which offer reasonably high electrical conductivity and chemical resistance are also subject to considerable back-diffusion and electromigration of hydroxyl ions, the rate of which increases with the concentration and temperature of the catholyte.
A second type of cell for chlorine and caustic soda production is the mercury cell in which electrolysis of the brine results in production of a sodium amalgam at the cathode and chlorine at the anode. The amalgam is reacted with water, producing a salt-free solution of caustic soda together with hydrogen. In this method a diaphragm is not required because caustic soda solution is formed in a separate piece of apparatus or compartment from the one containing brine solution and chlorine.
In one embodiment of a mercury cell, purified sodium chloride brine is fed into a slightly sloped horizontal trough on the bottom of which the cathode mercury flows concurrently with the brine. Above the mercury and within the brine are horizontal anodes of graphite or platinum-, or platinum family-coated titanium. These anodes are suspended from gas tight covers of the trough. Current is supplied to the anodes by rods suspended from, and sealed in, holes in these covers.
The brine is confined within the trough by mercury syphons at the ends of the trough. Typically the concentration in the cell is reduced from 315 to 275 g/l NaCl and the brine leaves the trough by an overflow. Conventionally, it is fed to the trough by a valve and rotameter. After leaving the cell, the brine is dechlorinated by a combination of HCl addition and vacuum and air stripping, resaturated, purified and returned to the cell with its pH at about 7.0.
Electricity is led into the mercury by connections to the steel trough bottoms with bus-work. Anode rods that project above the cover, and usually have to be vertically adjustable, are connected to the main bus-work by pigtails, clamps, soldered rods or similar hardware. To stop operation of each trough, short circuit switches that connect the anode bus-work to the cathode buswork are usually provided.
The covers, sidewalls, end boxes, seals and, at times, most of the bottom of the trough are covered with corrosion resistant material, usually hard rubber. The life, before repair or replacement becomes necessary, hardly ever exceeds five years and is often much less. This assemblage of components is commonly called the primary cell.
The mercury flowing out of the primary cell contains sodium, plus impurities such as calcium, magnesium and iron that may not have been fully removed in the prepurification of the brine. To remove the sodium, thus making caustic soda, the mercury is washed with distilled water. This operation is performed in contact with graphite, leading to the production of caustic solution, hydrogen gas and relatively sodium-free mercury, which is pumped back to the inlet box of the primary cell. The apparatus for this is conventionally called the secondary cell or denuder. In present practice, it is either a horizontal trough with graphite grids, or a short tower with graphite packing. If it is a trough, it is arranged either alongside or under the primary cell; if it is a tower it is usually arranged on the discharge end of the primary cell, with the pump underneath the tower and a long pipe under the primary cell that conducts the mercury back.
To understand the function of the mercury cathode in a mercury chlorine-caustic cell, consider the electrolysis of a sodium chloride solution between a steel cathode and a graphite anode. Chlorine is evolved at the anode and hydrogen at the cathode, while caustic soda is formed simultaneously at the latter, according to the reaction: EQU 2NaCl + 2H.sub.2 O = 2NaOH + Cl.sub.2 + H.sub.2
with no separation of the anolyte and cathode, the following secondary reactions occur: EQU 2NaOH + Cl.sub.2 = NaCl + H.sub.2 O EQU 3 NaOCl = NaClO.sub.3 + 2NaCl EQU C (Graphite) + 2NaCOCL = 2NaCl + CO.sub.2 EQU Cl.sub.2 + H.sub.2 = 2HCl (Explosion)
Obviously such a cell does not serve the purpose of producing caustic soda and chlorine. On the other hand in a mercury cell, and utilizing a mercury cathode, if the mercury is relatively pure, hydrogen will not be evolved on the cathode preferentially to the discharge of sodium, and the mercury will become sodium amalgam, in accordance with the following: EQU 2NaCl .fwdarw. 2Na(Hg) + Cl.sub.2
because the hydrogen overvoltage on a mercury surface is higher than the voltage required to deposit sodium into such a surface. The overvoltage is the voltage of an electrode above that theoretically required to discharge a gas on its surface.
If the mercury contains more than a few tenths of one percent of sodium, or traces of magnesium, nickel or similarly low hydrogen overvoltage metals, hydrogen and caustic soda rather than sodium amalgen will form to a greater or lesser extent. When this happens, the current efficiency drops, graphite consumption increases and the chlorine gas in the cell, or in the uncondensable gas left after most of the chlorine is liquefied, becomes explosive because of hydrogen admixture.
The reverse effect is desired when making caustic and hydrogen from amalgam. If clean sodium amalgam is placed in a beaker and water or caustic is poured over it, little or no reason takes place, because hydrogen does not readily discharge from a mercury surface. If a piece of graphite is partly dipped into the mercury, hydrogen bubbles can be observed rising from the graphite very near the mercury surface, the water or caustic become more alkaline and the amalgam is denuded of its sodium. Thus, under operating conditions the denuder is a short-circuited battery in which the amalgam is the anode and the graphite the cathode.
The process produces a pure concentrated caustic, normally about 50%, compared to about 11% concentration for the diaphgram cell. However, the circulation of mercury, the exposure of the mercury surface to salt brine where it is always denuded to a greater or lesser extent, and other problems inherent in the technology have always forced the design into large, expensive, complicated equipment occupying large buildings and leading to inevitable mercury pollution from skimming operations and the like. Furthermore, mercury cells are extremely sensitive to impurities in the sodium chloride solution as these increase the denuding of the amalgam that takes place during electrolysis, resulting in a high and frequency explosive hydrogen content of the chlorine.
The problems of circulation of mercury may be avoided if the configuration is one in which the mercury is used as a membrane, one face of which is the cathode in the brine which undergoes electrolysis, the other face of which is in contact with the caustic soda solution. In this manner the deposition of sodium in the amalgam occurs simultaneously with denuding at the opposite face.
Attempts to achieve this have invariably led to a design wherein the mercury is either supported in syphon-like superimposed channels or on porous or woven materials, essentially on a diaphragm. When the mercury is supported in syphons, the long path that the sodium metal must travel through the mercury results in overconcentration of sodium at the cathode surface accompanied by hydrogen evolution on the cathode surface and into the chlorine and, if the mercury is supported on a diaphragm, the eventual result has always been that the resistance increase because gas bubbles become entangled in the pores. Furthermore, deposition of metallic impurities such as iron in the pores results in wetting of these impurities by mercury accompanined by mercury leaking through the diaphragm and being lost.
A variation in the mercury cathode cell is disclosed in U.S. Pat. No. 2,749,301. The mercury cathode is supported on a porous diaphragm of a woven plastic fabric or asbestos cloth. The brine flows beneath the cathode on the anode surface. A very large, and thus uneconomical, flow of brine must be pumped through the space between the anode and the diaphragm to avoid the blanketing of the diaphragm by gas bubbles. However, even with a high flow rate, bubbles of chlorine as well as hydrogen from the mercury layer above the diaphragm slowly become entangled in the diaphragm and reduce the efficiency of the process.
An object of the present invention is to provide novel processes and apparatus for the electrolysis of alkali metal solutions. It is also an object of this invention to overcome certain deficiencies which exist in presently used electrolysis cells and processes.
These and other objects will be set forth more fully in the following detailed description.